Growth of the pulmonary capillaries by angiogenesis is essential for alveolarization, and disruption of pulmonary angiogenesis contributes to the pathogenesis of bronchopulmonary dysplasia (BPD), the most common complication of premature birth. However, the molecular pathways that orchestrate pulmonary angiogenesis during development and disease are not fully understood. This gap in knowledge continues to confound efforts to develop targeted therapies to treat lung diseases caused by impaired angiogenesis, including BPD. Nuclear factor ?-B (NF?B) is a key regulator of inflammation, and growing evidence suggests that NF?B regulates angiogenesis in wound healing in cancer. In contrast, little is known about the role of NF?B in the developing lung. We recently demonstrated that NF?B is essential for alveolarization. NF?B is constitutively active in the neonatal, but quiescent in the adult lung, and inhibiting NF?B disrupts pulmonary angiogenesis and alveolarization in neonatal mice, but has no effect in adults. Blocking NF?B in neonatal primary pulmonary endothelial cells (PEC) impairs survival, proliferation, and in vitro angiogenesis. Activation of NF?B in the PEC corresponds to the period of angiogenesis in the developing lung, peaking at the onset of alveolarization, and decreasing to low levels by mid-alveolarization. However, the capacity for NF?B to be activated in the PEC is not intrinsic to the cell's maturation, as conditioned media obtained from early alveolar lung organ culture (EA-LCM), robustly activates NF?B and enhances migration in adult PEC. Global proteomic analysis to compare the early alveolar and adult lung secretomes identified factors secreted only by the early alveolar lung (EAL). Studies to examine the effect of neutralizing each of these proteins on the ability of the EA-LCM to promote NF?B activation and migration, allowed the identification of two putative activators of NF?B in the early alveolar lung: transforming growth factor- induced protein (TGFBI) and alpha-fetoprotein (AFP). Both factors are expressed in the EAL in vivo but minimally expressed in the adult lung, and both are suppressed by injuries that disrupt alveolarization. Therefore, our data suggest the overall working hypothesis that factors uniquely secreted by the EAL promote lung angiogenesis and alveolarization by activating NF? B in the pulmonary endothelium by addressing two inter-related specific aims. In Aim 1, we will examine if TGFBI and AFP secretion by the EAL induces a pro-angiogenic phenotype in the PEC via an NF?B-dependent pathway, and explore if TGFBI and AFP activate common or complementary patterns of gene expression. Aim 2 will utilize in vivo gain and loss of function strategies to determine if disrupting TGFBI- or AFP-mediated activation of NF?B impairs alveolar growth during development and injury. The successful completion of these studies will establish a novel, physiologic role for NF?B in the developing lung, and identify new angiogenic factors that can be directly translated into therapeutic strategies to promote lung growth and regeneration in diseases marked by impaired pulmonary angiogenesis.